Boosting pump-and-treat with electro-osmosis

Lawrence Livermore National Laboratory developed and is deploying a low-energy technology that serves as the first step in a Phased Source Remediation (Tech ID 2923)—an approach that seeks to lower remediation costs by first using less expensive technologies for starving a plume at its source before turning to progressively higher energy approaches. Electro-osmosis, in combination with hydraulic pumping, is being used to increase the rate of groundwater flow through extraction wells to a portable treatment unit on the surface, which is removing chlorinated hydrocarbons by air-stripping with activated carbon adsorption. In this Accelerated Site Technology Deployment project, investigators are assessing how electro-osmosis can be used to enhance a pump-and-treat remediation. The Subsurface Contaminants Focus Area is helping to fund this ASTD project.



LLNL’s Environmental Restoration Division is deploying electro-osmosis at a site near a helicopter pad and across from the National Ignition Facility, which is under construction. Fine-grained, low-permeability sediments in the saturated zone are contaminated with chlorinated hydrocarbons, primarily TCE, at relatively high concentrations from 5 to 10 ppm. This environment is difficult to remediate due to the clay soil’s retention of groundwater. The average flow rate to wells in the area is less than 0.5 gallons per minute. Electro-osmosis’ role is to overcome the soil’s tight grip on groundwater and increase the rate of groundwater flow toward cleanup.

Investigators have applied an electric field below ground through an array of nine electrode-bearing groundwater wells. Pore water, which is locally positively charged in the vicinity of negatively charged clay mineral surfaces, is induced to flow away from the positively charged anodes wells located on either end of the array toward the extraction wells in the center of the array that contain the negatively charged cathodes.

The system also includes buffering tanks at the surface to maintain pH neutrality during electrolysis reactions at both types of electrodes. pH control is necessary to prevent the formation of mineral deposits at the cathodes and acidification of the sediments near the anodes, which would cause a corresponding reduction in electro-osmotic conductivity.

After the use of electro-osmosis to expedite the extraction of water through wells, the water is sent to the portable treatment unit where the chlorinated hydrocarbons are removed. The water is then returned to the subsurface through the anode wells. Support equipment includes submersible pumps, water-level sensors, and flow meters for each well, along with a manifold assembly that directs the flow of water between extraction wells, pH adjustment units for the cathode and anode arrays, a water treatment facility, and injection wells. Geokinetics International, Inc. of Berkeley, California, supplied equipment and support for the project.

Preliminary results indicate that electro-osmotic pumping is leading to a measurable increase in contaminant extraction rates. Analysis of these initial data, coupled with flow and transport modeling, suggest significant reductions in the time required to remediate the site as opposed to an exclusive pump-and-treat approach. Currently, the system is being upgraded to allow for more reliable long-term operation.

For more information about this project, contact Bob Bainer at bainer1@llnl.gov.